Preheat oven to 400F and grease a 9x13 inch baking dish.
Add rice, French onion soup, consommé, and drained mushrooms to dish and stir to combine.
Place butter slices evenly across the top.
Cover tightly with aluminum foil.
Bake covered for 30 minutes without opening.
Remove from oven, uncover, and stir to distribute melted butter.
Return to oven uncovered for 30 minutes until top is golden.
Let rest 10 minutes, then fluff with a fork before serving.